This invention generally relates to a system and method of storing and delivering noble gas propellants. This invention specifically relates to a system and process for storing and delivering noble gas propellants for an ion propulsion system.
Ion propulsion systems are projected to play an increasingly important role for future planetary exploration, and for commercial and military earth orbit satellites. These systems produce larger specific impulses than their chemical thruster counterparts, yielding more efficient, lighter-weight propulsion systems.
Currently, noble gases (e.g., krypton, xenon) are favored propellants for electric or ion propulsion systems, in part, because they are non-corrosive and generally inert. Xenon is popular for such systems because it features (i) a low ionization potential and (ii) a large molecular weight of 131.3 g/mole.
In present ion propulsion systems, a controlled flow of noble gas (e.g., krypton, xenon) is delivered at low pressure to a thruster from a compressed gas storage vessel. The controlled flow of gas at low pressure (e.g., 20 pounds per square inch gauge ("psig")) from storage at high pressure (e.g., 1200-3000 pounds per square inch absolute ("psia")) is accomplished by a combination of pressure regulators, control valves and flow controllers. The flow rate is generally maintained at between about 5-60 standard cubic centimeters per second ("sccm") for current satellites. The flow rate for xenon would be approximately 0.5-6.0 milligrams per second ("mg/s"). Larger flow rates would be required for larger spacecraft or for planetary exploration.
Delivery systems for ion propulsion thrusters are generally comprised of: (i) a flow control(FC) device and (ii) a compressed gas storage system to store the gas and regulate gas pressure.
The flow control device is connected to the thruster, and the compressed gas storage system (including an upstream pressure management system (PMS)) regulates the pressure to the required flow control feed pressure. A series of pressure regulators, valves (latching, explosive, solenoid), flow restrictors, and heaters are used to deliver the noble gas from the storage system through the flow control device and to the thruster.
The FC device frequently is a thermally operated capillary tube flow restrictor, requiring 100-140 Watts. By adjusting the temperature of the gas, and hence its temperature-dependent viscosity, the flow rate of the gas flowing through the device is controlled. Other FC devices employ a dual solenoid valve assembly separated by a small ullage volume. As the solenoid valves are alternately opened and closed, the noble gas in the ullage fills and empties much like a peristaltic pump.
Compressed gas storage systems face inherent engineering challenges. First, they simultaneously must maintain high storage pressures while providing a regulated release of gas at lower pressures (e.g., 20 psig). Second, as the gas is used and the pressure of the vessel decreases, additional hardware and/or controls must be used to maintain a consistent flow rate. Third, these storage systems can be dangerous because the stored energy of the compressed gases may be equivalent to several pounds of TNT.
Another difficulty with compressed gas storage systems arises in space-based applications where the noble gas exists in multiple phases. This is problematic because gas-liquid two-phase systems are difficult to separate in zero-gravity, necessitating separation of the phases and/or a pre-expansion of the two-phase mixture in order to ensure gaseous delivery to the FC device.
For example, when exposed to the cold temperatures of outer space, xenon condenses to a liquid phase resulting in a two-phase system. Xenon has a critical temperature of 290 K which falls in the temperature range experienced by gas storage systems on orbiting satellites, e.g., 90 K to 450 K. The exact range depends, in part, on the insulation and temperature controls employed. The volume of compressed xenon, loaded on earth at a nominal 25.degree. C. to pressures of about 1000 psia or greater, is about 0.9 cm.sup.3 /g or less, and the critical volume of Xe is 0.9 cm.sup.3 /g.
A further difficulty is that gas-liquid two-phase systems are unreliable in providing a noble gas to an ion propulsion thruster, which is exacerbated by the sensitivity of ion propulsion systems to propellant flow variations. Another disadvantage is that two-phase storage systems may require venting of the noble gas, both prior to launch and in space. Venting is an expensive practice because satellite systems may use several thousand pounds of noble gas (e.g., xenon, is currently at $1,000 per kilogram or more). Moreover, venting adds complexity to the vessel and system design, further increasing costs. In space, venting may be necessary to control the temperature and pressure of the two-phase fluid. This results in extra payload, launch costs and wasted propellant.
Additionally, the engineering design of gas-liquid two-phase systems requires that the thermal control elements (i.e., heaters and coolers) contact the fluid regardless of the fluid level in the storage vessel, which increases engineering design costs. Because even partial vaporization of the liquid in the storage vessel at pre-launch would create a high pressure "bomb" of several thousands pounds of pressure, protective thicker-walled tanks are required, which increases the weight penalty over other gas storage systems.
Nonetheless, two-phase storage systems have continued to be used because of the spatial advantage of storing noble gas in liquid form.
In short, two-phase storage systems have many disadvantages for space-based ion propulsion system applications. They are complicated and expensive, demand special temperature controls, have large power requirements (e.g., 100+W), require special safety features, and must be designed and built specially for operation in an outer space environment (e.g., factoring in orbit parameters, system geometry, etc.). These systems are more difficult to design because of the two-phase separation and the necessity of pre-expansion of the two-phase mixture to assure proper vapor delivery to the FC device. And, for pre-launch logistical reasons, these systems require venting, this further increasing design complexity and expense.